Field of the Invention
An embodiment of the invention relates to the field of DSL (Digital Subscriber Line) communications, and in particular, to the use of noise reference signals to reduce noise interference on DSL transmission lines.
Description of Related Art
DSL deployment began in the mid 1990s. There are now over 300 million DSL transmission lines (or simply “DSL lines”) worldwide. DSL is designed to provide high bit rate data communication by utilizing high frequency signals on unshielded telephone lines. The telephone lines were originally designed for voice communications, not high frequency data transmissions, and are inherently susceptible to noise interference (or simply, “noise”) in the high frequency bands used by DSL.
Noise interference on DSL transmission lines (that is, on unshielded telephone lines carrying DSL data traffic) is responsible for significant performance degradation in DSL deployments throughout the world. It is not uncommon for the majority of the lines in a typical DSL network to suffer significant data rate loss as a result of noise interference. In most DSL networks there are many lines that suffer well over a 50% loss in data rates as a result of noise. Numerous noise sources impair DSL transmission including AM radio transmissions, impulse noise from a variety of sources, harmonic noise from computers and televisions, interference from other digital communications protocols such as T1, and crosstalk from other DSL transmission lines.
As early as the mid 1990s DSL designers have recognized the negative effects of noise interference and have sought to mitigate the resulting performance degradation. U.S. Pat. No. 5,995,567 entitled ‘Radio Frequency Noise Cancellation’, issued in 1997 describes the use of a noise reference signal and the well known LMS (Least Mean Square) cancellation technique to reduce noise interference on the DSL transmission line.
Over the years numerous attempts have been made to utilize noise reference signals to reduce noise on DSL lines and thereby improve DSL data transmission performance (higher bit rate, longer transmission range and/or fewer transmission errors). Commonly available noise reference signal sources include the common mode signal on a DSL line and signals derived from other telephone lines in the vicinity of the DSL line. These line-derived (and other similar) noise reference signals have two sub-optimal characteristics that might result in poor noise cancellation performance.
First, telephone line derived noise reference signals could contain some residual DSL data signal energy as a result of telephone line crosstalk. Noise cancellers work to minimize correlated energy on the DSL line and as a result will often work to ‘cancel’ the DSL data signal thereby degrading DSL modem performance. Hereafter, this effect is referred to as “data signal cancellation”.
Second, these noise reference signal sources often have a complex combination of many noise signals. Various noise sources will in general couple into the noise reference signal and the DSL data line differently. For example, some noises that are not present on the DSL line may couple into the noise reference signal. In such cases a conventional cancellation approach (such as LMS) that simply works to reduce the overall broadband noise energy may achieve poor results in terms of achieving any DSL transmission performance improvement. For example, an LMS canceller may focus on a large energy narrowband AM noise, achieving modest bit rate enhancement at the AM frequency band, while generating greater noise deterioration in other bands. In general, broadband energy-reducing algorithms do not have the ability to optimize the frequency band cancellation focus in areas that would result in optimal DSL data transmission performance enhancement.
Various solutions have been proposed to overcome the problems that result from noise reference signals of this nature. Each of the proposed solutions has certain limitations and deployment issues.
Several of the proposed solutions involve techniques that utilize commonly available telephone line derived noise reference signal sources. Use of such readily available noise sources is preferred in order to facilitate wide deployment.
None of these techniques, however, has provided an effective method for mitigating the detrimental DSL data signal cancellation effect that results from the residual DSL data signal often found in telephone line derived noise reference signal sources. Cioffi et al., in U.S. Pat. No. 5,995,567, describe training the noise canceller only during periods of no DSL data transmission in order to resolve this problem. This technique is proposed for use in ping-pong time duplex Asynchronous DSL (ADSL) systems, which have very low applicability in the current world wide deployment, since commonly used Frequency Division Duplex (FDD) and echo-cancelled DSL technologies do not have repeating silent periods as required by U.S. Pat. No. 5,995,567. Magesacher, “Analysis of Adaptive Interference Cancellation Using Common-Mode Information in Wireline Communications” further describes the problem of DSL data signal cancellation and concludes that noise cancellers using a common mode line derived noise reference signal deteriorate DSL performance when adapting during data transmission (and hence are not readily applicable to commonly deployed DSL technology).
Amrany et al., U.S. Pat. No. 6,999,504, describe the use of the DSL line common mode signal as a noise reference signal. While use of this signal is desirable due to its availability on all DSL connections, Amrany do not address the problems of data signal cancellation or non-optimal frequency band focus. In practice, these problems limit the use of a common mode signal as a noise reference signal to achieve DSL transmission line performance improvement when used with conventional cancellation techniques.
Other approaches describe techniques that improve the ability of a noise canceller to handle the many types of noise found in line derived noise reference signal sources (such as multiple sources of AM, impulse, harmonic, and/or crosstalk, noise). Cioffi et al. in PCT published patent application, publication number WO 2008/045332, propose analysis of the noise reference signal and adjustment of cancellation parameters based on the class of noise present. Techniques described therein include storing sets of coefficients, applying cancellation thresholds, and incorporating demodulation feedback. These approaches assist a noise canceller in optimizing cancellation for the class of noise present, but do not directly address the issue of data signal cancellation as a result of a residual DSL data signal in the noise reference signal.
Other techniques, set forth below, propose the use of a more optimal noise reference signal source. Approaches of this nature have limited applicability due to the limited amount of noise that can be cancelled from the proposed reference signal sources and/or the limited availability of the proposed reference signal sources.
Fischer, et al., in U.S. Pat. No. 7,003,094, describe using an AM radio receiver to generate a suitable AM noise reference signal that would not be subject to the problems associated with line derived noise reference signal sources. This approach is effective for the cancellation of noise from transmissions of one or more AM radio stations. However, the use of AM receivers as noise reference signal sources does not provide a large improvement to the aggregate performance of a DSL network because AM transmission noise may not be one of the larger noise impairment sources.
Bingel, et al. in U.S. Pat. No. 6,477,212, describes a noise reference signal derived from a separate detection device such as an antenna. In practice, a detector will not generate a well correlated noise reference signal if it is decoupled from the line signals to the extent that the problems associated with line derived noise references are not present. Crosstalk and much of the line coupled impulse and harmonic noise will not be well represented in a line isolated detector. Approaches of this type are primarily suited to cancellation of AM and amateur radio broadcast signals. Improvement in only these types of noise has limited aggregate benefit to a DSL network.
Vectored DSL noise cancellation is an approach that provides effective cancellation of multiple DSL crosstalk sources by providing a cancellation device with an ideal reference signal from other DSL transmitters that are likely to have a crosstalk effect. While this is an effective method to reduce DSL crosstalk interference, it is only applicable to new systems with heavily modified architectures that support sharing transmit signal sources between DSL Access Multiplexor (DSLAM) modems. Furthermore, vectored DSL does not address cancellation of other external noise sources such as AM radio transmission or impulse noise at the Customer Premises Equipment (CPE) end of the DSL connection.
Shah et al. in Patent Cooperation Treaty (PCT) application, publication number WO 2004/027579, describe the use of multiple receivers with interconnection such that received data signals from one line can be used to cancel crosstalk on another line. This approach requires an interconnection path from multiple receivers and is therefore not applicable to the majority of DSL network installations.
A significant advancement in the art would be accomplished by an embodiment of an invention that could overcome the problems resulting from a residual DSL data signal present in a noise reference signal and/or that could optimize the frequency band cancellation focus of a noise canceller such that readily available line derived noise reference signals could be used to improve the aggregate performance of a DSL network.